Cr(?) Adsorption At Clay Interfaces And Mechanisms For Cr(?) Reduction By Hydroxyl-containing Organic Molecules And Cysteine

Chromium(Cr)is a major contaminant that enters into soils and water systems via industrial activities including leather tanning,steel manufacture,mining metallurgy,chemical dyeing,electroplating and agricultural production such as sewage irrigation,use of sludges and application of chromium-containing fertilizers.The mobility and biological toxicity of chromium are closely related to the valence state.Cr(?)species has high mobility and strong biological toxicity,and is identified as one of the key carcinogens.A large number of studies have shown that adsorption at mineral surfaces with considerable adsorption characteristics represents an efficient and low-cost approach to control Cr(?)contamination,while toxic Cr(?)remains.In contrast,Cr(III)is much less toxic and mobile.Cr(?)reduction at mineral interfaces to Cr(III)is a more effective method than adsorption for control of Cr(?)contamination.As early as 1998,Teppen stated that computational chemistry will promote development of soil chemistry in his article entitled"Computational Chemistry in the Future of Soil Chemistry".It can analyze the interaction and internal mechanism from the micro-atomic scale,and is a favorable means to solve the problem of micro-structure mechanism.Computational chemistry methods such as density functional theory(DFT)calculations have been used in this thesis.Herein,adsorption behaviors of Cr(?)oxyanions onto different interfaces of clay minerals are firstly studied and influence factors such as hydration and pH changes are discussed.The migration processes of different Cr(?)oxyanions and their transformation mechanisms are further explored.Meanwhile,in order to have a more comprehensive understanding of organic molecules with obvious structural differences and organic vs.biological reduction mechanisms,the reduction processes of Cr(?)oxyanions by hydroxyl-containing organic molecules(alcohols and vitamin C)and thiol-containing cysteine are investigated,focusing on the analyses of reaction paths,energy barriers,reduction mechanisms and product distribution.The main findings are summarized as follows:(1)Basal surfaces of clay minerals contain silica tetrahedral and alumia octahedral surfaces.Kaolinite is used as model of clay minerals to investigate adsorption of Cr(?)oxyanions at tetrahedral and octahedral interfaces.Both interfaces can adsorb Cr(?)oxyanions,while adsorption mechanisms have clear differences.When Cr(?)oxyanions(Cr₂O₇^2-and CrO₄^2-)adsorb at tetrahedral surfaces,K⁺counterions serve as bridge for Cr(?)oxyanions and tetrahedral surfaces,and adsorption of Cr(?)oxyanions is compromise of electrostatic attraction and repulsion.However,when Cr(?)oxyanions adsorb at octahedral surfaces,they can interact directly through hydrogen bonds in the form of O_sH···O_Cr,and Cr(?)oxyanions function as bridge of octahedral surfaces and K⁺ions that form direct K-O_Cr bonds with octahedral surfaces.Introduction of water solvent causes slight structural perturbations to Cr(?)adsorption configurations and owing to strong competition from water molecules,interactions among K⁺ions,Cr(?)oxyanions and kaolinte surfaces are attenuated,while facilitates translational motion of Cr(?)oxyanions along clay surfaces.At the same time,adsorption stability is altered significantly by pH variations,and stability trends at different pH ranges are deciphered by dominant interaction force with clay surfaces.Stability trends at tetrahedral surfaces follow as“pH<PZC(point of zero charge)”<“pH?PZC”<“pH>PZC”for both Cr₂O₇^2-and CrO₄^2-whereas at octahedral surfaces follow as“pH<PZC”<“pH?PZC”<“pH>PZC”for Cr₂O₇^2-and“pH?PZC”<“pH<PZC”<“pH>PZC”for CrO₄^2-.In addition,charge transfers when pH>PZC and pH<PZC are more obvious than when pH?PZC,and kaolinite surfaces serve as electron reservoirs and can gain or lose electrons as required.(2)Transport and transformation between Cr(?)dimer and monomer(Cr₂O₇^2-and CrO₄^2-)become possible at clay surfaces.Compared with Cr₂O₇^2-,CrO₄^2-always has superior stability and should be dominant Cr(?)species at clay interfaces.When Cr(?)exists in the CrO₄^2-form,removal from wastewater by clay minerals becomes more efficient,and basic conditions favor Cr(?)immobilization.However,when Cr(?)exists in the Cr₂O₇^2-form,Cr(?)transport at clay interfaces becomes more efficient and facilitates separation from clay surfaces,which favors recycle and reclamation.Kinetic studies of the rate-decisive hydrolysis step indicate that when at tetrahedral surfaces,hydrolysis process is greatly affected by pH and Cr₂O₇^2-transformation to CrO₄^2-is kinetically blocked due to the high energy barrier at pH?PZC,which thus disfavors Cr(?)immobilization.Either elevating or lowering the pH kinetically facilitates transformation and enhances Cr(?)adsorption at clay interfaces,which are derived from the enhancement of product stability by K⁺when the pH increases and the weakening of the stability of Cr₂O₇^2-when the pH decreases,respectively.However,rhe hydrolysis process of the octahedron is less affected by pH.The increase in the energy barrier when pH>PZC is due to the enhancing of hydrogen bond between the octahedron and Cr₂O₇^2-which hinders the hydrolysis.Their results provide a basis for the study of reduction reactions.(3)For clay minerals,edge surfaces have a relatively small proportion while show superior reactivity.CrO₄^2-adsorbs stabby at edge surfaces of montmorillonite,for both Al³⁺and Mg²⁺sites.Cr(?)reduction occurs through proton transfers from alcohol hydroxyls and reaction is divided into two stages,as Cr(?)?Cr(IV)and Cr(IV)?Cr(II)/Cr(III).Reduction causes Cr(?)to be efficiently transformed to less toxic Cr(II),Cr(III),Cr(IV),and is largely determined by ?-conjugation development.Ethanol and ethanediol(representatives of monohydric and polyhydric alcohols)correspond to distinct reactivities and mechanisms.Ethanediol can use two equivalents to complete Cr(?)reduction just as ethanol,or only one equivalent.In addition,active sites(Al³⁺and Mg²⁺)are an important factor to affect Cr(IV)reduction.Energy barriers(E_a)for the most preferred paths are moderate under all conditions and alcohols are efficient for Cr(?)reduction.Cr(II)is the dominant product at Mg²⁺site with high-coverage ethanol or ethanediol and E_a values are 92.7 and 104.2 kJ/mol;Cr(III)constitutes the major product at Al³⁺and Mg²⁺sites with low-coverage ethanediol and E_a values are 104.6 and 72.4 kJ/mol;Cr(IV)accounts for the largest proportion under these conditions:Al³⁺site with different ethanol coverages,Al³⁺site with high-coverage ethanediol,and Mg²⁺site with low-coverage ethanol with E_a values of 75.1,117.1,88.1 and 2.6 kJ/mol,respectively.(4)The functional groups of vitamin C(VC)are mainly hydroxyls in the various forms,and non-hydroxyl functional groups also exist.When VC is used for Cr(?)reduction and Cr(V),Cr(IV),Cr(III)and Cr(II)may appear as products.The four reducing functional agents-CH₂OH,-CHOH-,HOC=COH and-HC-CH-are converted to-CH=O,O=C-C=O and-C=O-,-C=C-,respectively.Among them,-CH₂OH,-CHOH-and-HC-CH-are main reducing functional agents in the first stage,and energy barriers are 115.7,92.1 and 109.5 kJ/mol at the Al³⁺site and 52.9,62.8 and46.8 kJ/mol at the Mg²⁺site,respectively.Energy barrieres of Cr(?)reduction at the Mg²⁺site is merely half as those at the Al³⁺site,and promotion effects of Mg²⁺/Al³⁺substitution have been cooroborated.Hence,Mg²⁺site becomes the focus for the second stage.HOC=COH group is preferred in the second stage,and whether for one or two VC molecules to be used,energy barriers of the dominant path is lower than65.0 kJ/mol.The whole Cr(?)reduction process occurs favorably and Cr(III)with high stability beomces dominant.As for other three reducing groups,energy barriers for Cr(IV)reduction are as high as 140.9?201.6 kJ/mol,while are less than 120.0kJ/mol for VC intermediates(VC_HC-CH,VC_CHOH and VC_CH2OH)formed in the first stage.These fully indicate that the reduction characteristics of different groups are significantly different and have relative advantages.(5)Cysteine(Cys)is a common amino acid,and its interaction with Cr(?)occurs inside organisms.The thiol group is the unique reducing agent of Cys.Cr(?)can reduce not only by the alcohol-like oxidation of cysteine(1Cys),but also by the S-S dimerization of cysteine(2Cys)with formation of cystine.Active sites(Al³⁺and Mg²⁺),internal structure of Cr oxyanions and choice of Cys reduction mechanism affect reduction processes and distribution of products.Among them,the influences of active sites(Al³⁺and Mg²⁺)are mainly refected in the first stage.Cr(?)reduction always occurs via stepwise mechanism(Cr(?)?Cr(V)?Cr(IV))at the Al³⁺site,while is inclined to proceed via concerted mechanism(Cr(?)?Cr(IV))at the Mg²⁺site.Mg²⁺/Al³⁺substitution significantly lowers energy barriers.Cys reactions mainly affect the second reduction stage.When Cr(IV)is reduced by alcohol-like oxidation of Cys,concerted mechanism dominates and energy barrier is relatively high.However,when Cr(IV)is reduced by S-S dimerization of Cys,stepwise mechanism is preferred and energy barrier is lowered significantly.Comprehensive analyses indicate that energy barriers are 124.6,115.9,117.3 and 71.9 kJ/mol for the dominant paths of Al-1Cys,Al-2Cys,Mg-1Cys and Mg-2Cys,respectively.For both Al³⁺and Mg²⁺site,Cr(?)reduction prefers to the S-S dimerization of Cys,and the difficulty of Cr(?)reduction by Cys is alleviated by Mg²⁺/Al³⁺substitution.Accordingly,Path 1 of Mg-2Cys system is the most favorable path.In summary,this thesis gives a thorough and systematic study on adsorption and transport characteristics of Cr(?)at clay surfaces at microscopic scale.It proves that the CrO₄^2-rather than Cr₂O₇^2-is the domiant form for Cr(?)at clay surfaces and thus provides the basis for subsequent Cr(?)reduction.Alcohols and vitamin C are two kinds of hydroxyl-containing organic molecules,and Cr(?)reduction by them reveals clear differences and impacts of other functional groups.Further analysis found that changing the reduction conditions(active sites of clay minerals,reducing agent type,reducing agent concentration,etc.)can selectively form target products such as Cr(IV),Cr(III)and Cr(II),and then realize the dual goals of Cr(?)pollution control and turning waste into treasure.Cr(?)reduction by cysteine further shows that the thiol group triggers Cr(?)reduction through different mechanisms.In addition,detailed electronic structure and vibration frequency analyses have been provided to guide experimental studies.These results significantly promote our understanding about Cr(?)adsorption by clay minerals,and chemical and bioreduction of Cr(?)that are critical to pollution control,and also provide theoretical support and scientific basis for exploring better adsorbent and reduction systems.